A magnetic fluid seal device is known as an example of a high-performance seal device that is clean and has long service life. This magnetic fluid seal device is widely used as a vacuum seal for introducing rotational drive force in a vacuum, a dust-proof seal for preventing bearing oil mists or the like from contaminating clean areas, or a gas seal or the like in various coating/etching processes or in manufacturing processes for semiconductors, liquid crystals, and the like in which there is a need for a maintenance-free shaft seal mechanism by which a clean atmosphere is obtained.
FIG. 5 is a sectional view showing the configuration of conventional example of a sealing device which uses a magnetic fluid (hereinafter referred to as “Prior Art 1;” see Patent Document 1, for example).
This magnetic fluid shaft seal device 50 has a cylindrical casing 52 on a wall 51 which forms a process chamber 60. In a state in which a relatively large amount of dust is present on an outboard side A, the magnetic fluid shaft seal device 50 maintains an inboard side L in a clean state, i.e., a dust-proof state, in which there is minimal dust.
The casing 52 surrounds a rotating shaft 53 extending through the inside and outside of the process chamber 60. An annular main pole piece 54 centered around the rotating shaft 53 and formed by a magnetic material, and an annular secondary pole piece 55 positioned on the right side of the main pole piece 54 and also centered around the rotating shaft 53 are housed inside the casing 52. An annular magnet 56 centered around the rotating shaft 53 is provided between the main pole piece 54 and the secondary pole piece 55. A concave groove 58 for accommodating a ring member 57 is formed between the main pole piece 54 and the secondary pole piece 55.
A slight gap is formed between an internal peripheral surface of the main pole piece 54 and an external peripheral surface of the rotating shaft 53, and between an internal peripheral surface of the secondary pole piece 55 and the rotating shaft 53, and a magnetic fluid is injected into each gap. A magnetic force line emanating from the magnet 56 forms a magnetic closed circuit through the main pole piece 54, the rotating shaft 53, and the secondary pole piece 55 and returning to the magnet 56, as indicated by the arrow B, and the magnetic fluid is focused at the magnetic force line to form magnetic fluid films 59.
Since the magnetic fluid shaft seal device 50 is configured as described above, environmental variations between the outboard side A and the inboard side L, e.g., pressure differences, variations in the gas atmosphere and amount of dust, and other variations, are maintained by the magnetic fluid films 59 and the ring member 57 in pressurized contact with the rotating shaft 53.
However, in the magnetic fluid shaft seal device 50 of Prior Art 1, impacts, vibrations, and other disturbances cause the magnetic fluid to scatter to the outboard side A or the inboard side L through the gap with the external peripheral part of the rotating shaft 53. When the magnetic fluid leaks out due to this scattering, the functioning of the magnetic fluid seal is compromised, and the outboard side A or the inboard side L becomes contaminated. Such leakage must therefore be prevented.
FIG. 6 is a sectional view showing the configuration of a magnetic fluid seal structure provided with a means for preventing leakage of magnetic fluid (hereinafter referred to as “Prior Art 2;” see Patent Document 2, for example).
This magnetic fluid seal 70 has a magnet 72 fixed to an internal peripheral part of a hub 71, and a pair of pole pieces (magnetic pole pieces) 73, 73 disposed in the axial direction on end faces of both sides of the magnet 72 in the axial direction thereof, and magnetic fluid 74 is retained at the internal peripheral edges of the pole pieces 73.
A magnetic ring 76 is fitted on an end portion (left end portion in the drawing) of a shaft 75 so as to face the magnetic fluid seal 70. The magnetic ring 76 is positioned so that the magnetic fluid 74 can make contact therewith, and is formed of a predetermined magnetically permeable body so that a magnetic circuit of the magnetic fluid seal 70 is formed. By this configuration, the magnetic fluid 74 is in close contact with the external peripheral surface of the magnetic ring 76, the opening between the shaft 75 and the hub 71 is sealed in pressure-resistant fashion, lubricating oil and the like are prevented from leaking to the outside, and penetration of dust and the like from outside a motor is prevented.
A leak prevention cover 77 for preventing the magnetic fluid 74 from scattering to the outside is also provided on the outboard side A of the magnetic fluid seal 70. The leak prevention cover 77 is configured so as to completely block the magnetic fluid 74 from the outboard side A, and an oil repellent agent 78 is applied to a wall face on the inside thereof in the axial direction, and to an axial end face of the magnetic ring 76.
In such a structure for a magnetic fluid seal, even when an impact, vibration, or other disturbance causes the magnetic fluid 74 to leak from the magnetic fluid seal 70, a leaked magnetic fluid 79 is first held back by a blocking wall surface of the leak prevention cover 77, and the surface tension of the oil repellent agent 78 then causes the leaked magnetic fluid 79 to form an oil droplet in a single mass without dissipating. The leaked magnetic fluid is then flung outward by centrifugal force from rotation and rapidly moved into a magnetic fluid reservoir 80.
In Prior Art 2, although magnetic fluid can be prevented from leaking, due to the structure of the seal, the magnetic fluid 74 cannot be injected into the portion for the pole pieces 73, 73 during assembly of the seal portion. The leak prevention cover 77 also obstructs injection of the magnetic fluid 74 from the outside after the seal portion is assembled in the device, and it is difficult to inject the magnetic fluid.
No provision is made for replacing seal components in Prior Art 1 and Prior Art 2, and component replacement is difficult in these seal devices.
The technique of Prior Art 2 is capable of sealing out only light dust or gas, and has the drawback of being incapable of sealing against mist or liquid.
An oil seal can seal against mist or liquid, but has the drawback of high torque.
FIG. 7 is a sectional view showing the configuration of a magnetic fluid seal device capable of sealing against mist or liquid (hereinafter referred to as “Prior Art 3;” see Patent Document 3, for example).
This magnetic fluid seal device 80 seals a gap between a non-magnetic housing 81 and a non-magnetic shaft 82 which is inserted into the housing 81, the housing and the shaft being two members which are assembled so as to be able to rotate relative to each other in concentric fashion.
The magnetic fluid seal device 80 is composed of a magnetic fluid seal member 80A formed by a permanent magnet 83 as a magnetic source magnetized in the axial direction, yokes 84 as a pair of pole pieces disposed on both sides of the permanent magnet 83, and a magnetic fluid 87 retained by magnetic attraction in a minute gap 86 between the yokes 84 and the housing 81 so as to form a seal via a sleeve 85 which is fixed to an internal periphery of the housing 81; and a labyrinth seal component 80B formed by a rotating body 88 fixed to the shaft 82 so as to rotate together with the shaft 82, and a labyrinth seal part 89 provided to the outer diameter side of the rotating body 88. The magnetic fluid seal device 80 is a labyrinth combination-type magnetic fluid seal device.
In the magnetic fluid seal device configured as described above, since labyrinth seal parts 89 formed by labyrinth seal components 80B are provided on both outer diameter sides of the magnetic fluid seal member 80A, even when small amounts of oil, water, or dust OL come toward the sealed side, since the rotating body 88 provided with the labyrinth seal part 89 also rotates when the shaft 82 rotates, the oil, water, or dust OL is shaken free by centrifugal force, the oil, water, or dust OL can be prevented from penetrating into the magnetic fluid seal member 80A, and it is possible to provide a seal against oil mist or grease from the inboard side L, and from oil, water, and other contaminants from the outboard side A.
However, in Prior Art 3, although a seal can be provided with respect to mist or liquid, the structure of the device makes it impossible to inject the magnetic fluid 87 to the pole piece 84 portion during assembly of the seal portion, the same as in Prior Art 1 and Prior Art 2. The rotating body 88 also obstructs injection of the magnetic fluid 87 from the outside after the seal portion is assembled in the device, and it is difficult to inject the magnetic fluid.
Like Prior Art 1 and Prior Art 2, Prior Art 3 also makes no provision for replacing the seal components, and component replacement is difficult.